Light-transmitting substrates provided with a light absorbing coating can be used as a color layer on or in front of lamps for general lighting purposes. The substrate may comprise, for example, a colored filter made of a piece of glass, which is flat or non-flat and which is designated to be placed on trajectory of light, said light being generated by a lamp. Such application is often used in outdoor lighting. Another example of a light-transmitting substrate is a lamp vessel that is placed over a light source of an electric lamp. Such electric lamps are predominantly used as indicator lamps in vehicles, for example as red-colored light source in red tail and brake lights of automobiles. Said electric lamps can also be used in traffic lights.
An electric lamp of the type mentioned in the opening paragraph is known from WO 01/20641 as filed by the present applicant.
The electric lamp according to WO 01/20641 is provided with an optically transparent, non-scattering, light-absorbing coating in which pigments are incorporated in a sol-gel matrix and which can resist temperatures up to 400° C. The sol-gel matrix in which the pigments are incorporated can reach a maximum layer thickness of about 500-800 nm when tetraethoxy silane (TEOS) is used as a sol-gel precursor and a maximum layer thickness of about 2-3 μm when methyltrimethoxy silane (MTMS) is used as a sol-gel precursor. The thickness of the light-absorbing coating, and in particular its critical layer thickness, is important. Actually, if the critical layer thickness is low, the light absorbing coating may crack and thus peel off. It is known from WO 01/20641 that addition of nano-sized silica particles in the light absorbing coating makes it possible to obtain thicker coatings which bond well to the lamp vessel.
However, the light absorbing coating of WO 01/20641 shows a relatively low scratch resistance. As a consequence, the coating may be scratched during the manufacturing process of the lamp, for example when clicking the lamp vessel on its cap. The coating may also be damaged during packing of the lamp, or mounting of the lamp in a luminary. This leads to a color point change, which may lead to a non-conform product.